110 



STEEL HIGHWAY BRIDGES. 



CHAP. III. 



called a Petit truss. Baltimore and Petit trusses are statically determinate for all conditions 

 of loading; are economical in construction and satisfactory in service, and have almost entirely 

 replaced the Whipple truss for long span bridges. 



The types of simple bridge trusses described above are those that are in the most common 

 use, although quite a number of other types of trusses have been used and abandoned. 



Beams and Plate Girders. For spans of, say, 30 ft. and under rolled beams are often used to 

 carry the roadway, while for spans from about 30 to 100 ft. plate girders are used for city bridges. 

 When the roadway is carried on top of the girders, the bridge is called a deck plate girder bridge, 

 and when the roadway passes between the girders, the bridge is called a through plate girder 

 bridge as in Fig. 19. 



(3) SMNG BRIDGE, CENTER BEARING fa) s mN6 & RIDGEt TURNTABLE BEARINQ 



FIG. 5. SWING BRIDGES. 



Swing Bridges. Swing bridges may be made of plate girders or trusses,, and may turn on a 

 center pivot as in (a), or on a turntable supported on a drum as in (b), Fig. 5. The center pivot 

 swing bridge has two spans continuous over the pivot support, while the turntable swing bridge 

 has three spans ordinarily continuous over the middle supports. 



Steel Arches. Steel arch bridges are made (i) with three hinges, (2) with two hinges, and 

 (3) without hinges, and may have solid webs, or spandrel or open webs. 



Cantilever Bridges. A cantilever bridge consists of two anchor spans, which support a 

 suspended or channel span. The shore ends of the anchor spans are anchored to the shore piers 

 and are supported on the river piers. 



Suspension Bridges. In a suspension bridge the roadway is supported by hangers attached 

 to the main cables. Stiffening trusses are placed above the plane of the roadway to assist in 

 distributing the live loads and for the purpose of increasing the rigidity of the structure. 



Simple truss bridges, beam and plate girder bridges, only, will be considered in this book. 



TYPES OF STRUCTURE. The types of structure for steel highway bridges as recommended 

 by the author are given in section 3, " General Specifications for Steel Highway Bridges," printed 

 in the last part of this chapter. 

 The following data will show present standard practice. 



Illinois Highway Commission. The types of highway bridge recommended by the commis- 

 sion are as follows: 



Concrete Bridges. For culverts requiring a waterway of 12 square feet or less, plain or rein- 

 forced concrete arch culverts or square culverts, reinforced concrete pipes or double strength cast- 

 iron pipe. 



For culverts having an area of more than 12 square feet, and for bridges having a span up to 

 30 ft., reinforced concrete slabs, plain or reinforced concrete arches. 



For spans of 30 ft. to 65 ft., reinforced concrete through or deck girders, plain or reinforced 

 concrete arches. 



For spans greater than 65 ft., plain or reinforced concrete arches. 



Steel Bridges. For spans of 12 ft. to 45 ft., steel I-beams; for spans of 30 ft. to loo ft., plate 

 girders or riveted pony trusses; for spans of go ft. to 160 ft., riveted trusses with parallel chords; 

 for spans of 160 ft. and more, riveted or pin-connected trusses with parallel or inclined upper chords. 



Iowa Highway Commission. The types of highway bridges recommended by the commission 

 are as follows: 



Concrete Bridges. Box culverts for spans up to 16 ft.; slab bridges for spans from 14 ft. to 

 25 ft.; arch culverts and bridges for spans of 6 ft. and over; girder bridges for spans of from 24 ft. 

 to 40 ft. 



Steel Bridges. Steel I-beams up to 32 ft. span; plate girders, 20 ft. to 80 ft. span; low truss 

 30 ft. to loo ft. span ; high truss 100 ft. span and over, riveted up to 140 ft. span. 



